(1) Field of the Invention
The present invention relates to a modified polyester fiber and a process for the preparation thereof. More specifically, the present invention provides a polyester fiber having a good cation dyeability and having excellent alkali resistance, heat resistance and spinnability, especially the spinnability at a high speed or ultra-high speed.
(2) Description of the Related Art
Since a polyester has various excellent properties, it is broadly used as a fiber. However, polyester is poor in dyeability and cannot effectively be dyed with a dye other than a disperse dye. Various proposals have been made to improve this poor dyeability. For example, there is known a process in which the main chain of a polyester is copolymerized with an isophthalic acid component containing a metal sulfonate group to render the polyester dyeable with a basic dye (see U.S. Pat. No. 3,018,272). According to this process, however, a large amount of diethylene glycol is formed in the polyester and the softening point is reduced to 220.degree. to 225.degree. C., and the polyester cannot be put into practical use.
As means for overcoming this, defect, Japanese Unexamined Patent Publication No. 48-66650, Japanese Examined Patent Publication No. 49-38037 and Japanese Examined Patent Publication No. 50-15274 propose a process in which a compound represented by the following formula: ##STR1## wherein R stands for a hydrogen atom, an alkyl group or the like, and M stands for an alkali metal or the like,
is copolymerized as a cation-dyeable component (modifier 1) in a polyester in the presence of an inhibitor for diethylene glycol formation. Now, a fiber composed of this copolymerized polyester is used as a cation-dyeable polyester fiber. However, this process is defective in that the spinnability of the obtained polyester is poor and even if spinning is carried out at a low take-up speed of less than 2500 m/min, troubles such as yarn breaking, contamination of the cap surface (deposition of the composed materials and the like) and increase of the pack pressure frequently take place. Furthermore, obtained fiber is poor in the alkali resistance and therefore, a product obtained by mix-weaving or mix-knitting this polyester fiber with an ordinary polyester fiber cannot be subjected to alkali weight-decreasing processing. Moreover, the heat resistance is poor, and a good false-twisted yarn cannot be obtained.
With a view to improving these poor alkali resistance and heat resistance of the conventional cation-dyeable polyester fiber, we made research and as the result, it was found that if a specific compound (modifier 2) including a compound represented by the following formula: ##STR2## wherein R stands for a hydrogen atom, an alkyl group or the like, and M stands for an alkali metal or the like,
is copolymerized instead of the modifier 1, the alkali resistance and heat resistance are greatly improved in the obtained polyester, and also the spinnability is greatly improved. However, if a fiber of this copolymerized polyester is dyed with a cationic dye, though the dye is completely absorbed, the dyeing finish is poorer than the dyeing finish attainable when the modifier 1 is copolymerized, and the brilliance is damaged and an obscure hue is obtained. Furthermore, a deep color cannot be obtained.